Polyimides have been widely used as materials for various industrial parts because of their excellent electric properties (low dielectric constants), heat resistance, mechanical strength, chemical resistance, formability and so on. Recently, polyimides for any use have been demanded to have better properties than ever before. Especially, polyimides used as materials for electric parts have been demanded to exhibit better electric properties (low dielectric constants).
As polyimides used as materials for electric parts, which have excellent properties, various polyimides are conventionally proposed. Non-Patent Document 1 discloses one of the polyimides. Specifically, Non-Patent Document 1 discloses that a straight-chain porous polyimide having pores of prescribed size is attained by removing silica particles from a straight-chain polyimide obtained via sol-gel reaction and having silica particles-dispersed phase, through hydrofluoric acid treatment of the straight-chain polyimide. The thus resulting porous straight-chain polyimide is considered to exhibit excellent electric properties (low dielectric constants) due to the presence of pores.
However, the inventors of the present invention have conducted an intensive study of the porous polyimide disclosed in Non-Patent Document 1 and found that although an improvement of electric properties (low dielectric constants) is confirmed, there is a risk of remarkable deterioration in mechanical strength because of relatively large pores present in the polyimide. The porous polyimide disclosed in Non-Patent Document 1 is obtained by removing the silica particles from the straight-chain polyimide having silica particles-dispersed phases (hereinafter, called as silica particles-containing polyimide in this paragraph). In the silica particles-containing polyimide, polyimide molecules and silica particles are merely mixed. As is obvious from the description of Non-Patent Document 1, there are no particular chemical bonds between the polyimide molecules and the silica particles.
Meanwhile, in JP-A-2007-246772, the present inventors have previously proposed a hyperbranched polyimide-based hybrid material that is obtained by using a mixture including a hyperbranched polyamic acid, which is obtained by reaction of an aromatic tetracarboxylic dianhydride with an aromatic triamine, and inorganic oxide particles. The hyperbranched polyamic acid present in the mixture is imidized to obtain the hyperbranched polyimide, thereby obtaining the hyperbranched polyimide-based hybrid material. Further, in Patent Document 1 (WO06/25327 pamphlet), the present inventors have proposed a hyperbranched polyimide-based hybrid material comprising an organic-inorganic polymer hybrid having a composite structure in which a polyimide phase and a silica phase are held together by covalent bond.    Patent Document 1: WO06/25327 pamphlet    Non-Patent Document 1: L. Jiang et al., “A methodology for the preparation of nanoporous polyimide films with low dielectric constants”, Thin Solid films, the U.S.A, Elsevier B.V., Jun. 3, 2006, Vol. 510, pp. 241-246